A stacker crane comprises a bridge-like overhead trolley which runs along laterally spaced overhead tracks and from which a mast structure projects downward that guides a load supporting carriage in up and down motion. The carriage can be brought to any desired position in the working space by a combination of its own up and down motion along the mast structure, rotation of the mast structure relative to the trolley, and horizontal movement of the trolley along the rails.
In a typical installation, a stacker crane is used with racks that provide storage stations arranged in horizontal rows and vertical columns. Usually such racks are located on opposite sides of an aisle along which the mast structure moves as it carries the carriage to a selected column of storage stations.
Because the load supporting carriage comprises a fork that projects from one side of the mast structure, the width of any aisle in which the crane has to perform loading and unloading operations has to be equal to the distance by which the carriage projects horizontally from the mast structure, plus the width of the mast structure, plus reasonable clearance. Ordinarily the width of a stacker crane mast structure is on the order of three feet, so that the mast structure accounts for a significant part of the needed aisle width. In turn, aisle width multiplied by the length of the aisle or aisles along which the stacker crane operates determines the number of square feet of floor space that must be dedicated to the operation of the crane. In general, the cost of a warehouse building is proportional to its floor area, regardless of whether floor area is utilized for storage or for access to stored materials, and this cost-floor area relationship applies not only to initial construction cost but also to such continuing costs as maintenance, taxes and insurance. Ultimately, therefore, a few inches more or less in the width of a stacker crane mast structure can have far reaching economic consequences.
In a stacker crane of the type here under consideration, the mast structure conventionally comprises a rigid mast and an intermediate mast. The rigid mast has its upper end connected to the overhead trolley and its lower end spaced a substantial distance above floor level. The intermediate mast moves up and down relative to the rigid mast. In a lowered position the intermediate mast provides a downward extension of the rigid mast that enables the carriage to be brought all the way down to floor level for loading and unloading. In a raised position of the intermediate mast the mast structure is clear of any persons and articles on the floor over which it travels.
Typical prior stacker crane mast structures are disclosed in U.S. Pat. No. 3,144,137 to Vallierre et al, U.S. Pat. No. 3,269,561 to DeLigt, U.S. Pat. No. 3,270,893 to Dechantsreiter, and U.S. Pat. No. 4,316,528 to Dechantsreiter.
In each of these, the intermediate mast surrounded the rigid mast and moved up and down along it. The carriage was in turn guided on the intermediate mast. In a lower range of carriage motion the intermediate mast projected below the rigid mast, nearly to floor level, while the carriage moved up and down relative to the intermediate mast; whereas in an upper range of carriage motion the carriage remained stationary relative to the intermediate mast, near the upper end of it, while the intermediate mast moved up and down along the rigid mast.
With the arrangement just described, the intermediate mast was necessarily wider than the rigid mast, in order to surround it for guidance up and down along it. However, the minimum width that the rigid mast could have was controlled by the requirement that it be stiff enough to resist excessive bowing along its length under the deflecting forces imposed upon it by load moments exerted through the fork of the carriage.
In the light of the present invention, it can be seen that the additional width contributed to the mast structure by the intermediate mast was required solely for guidance of the intermediate mast and offered no benefit with respect to strength or rigidity of the mast structure.
In some cases the effective width of the mast structure was further increased by the carriage, which was arranged in surrounding relation to the intermediate mast.